1. Field of the Invention
The present invention relates generally to a dielectric filter having a plurality of quarter wavelength type coaxial resonators and, more particularly, to a dielectric filter having a notch pattern for improving an attenuation characteristic of a high frequency band.
2. Description of the Related Art
Dielectric filters have been used for attenuating side frequency bands of a desired frequency band. Typically, the dielectric filters, such as a ceramic filter, include a dielectric block, which is made of a ceramic material, and a plurality of coaxial resonators formed in the dielectric block. Both a minimum insertion loss for the desired frequency band and an attenuation ratio for a side band of the desired frequency band should be accomplished in the dielectric filter. Moreover, the dielectric filter used in a device having a high frequency-band needs a more improved attenuation characteristic for the high frequency band, which is supposed to pass the dielectric filter. However, it is impossible that these conventional dielectric filters are minimized to be installed in a reduced size device because of the existence of an additional volume for a separate and individual notch hole formed in the dielectric filter.
FIGS 1A through 1C show a conventional dielectric filter, an equivalent circuit of the dielectric filter of FIG. 1A, and a diagram showing a representative characteristic of the dielectric filter of FIG. 1A. A dielectric filter 10 shown in FIG. 1A includes a dielectric block having an upper surface, a lower surface spaced-apart from and being parallel to the upper surface, and four walls perpendicular to both the upper surface and the lower surface. A plurality of coaxial resonators 10a, 10b, 10c includes respective through holes 11a, 11b, 11c formed in the dielectric block. A through hole 11d for a separate and individual notch hole is formed in the dielectric block parallel to and adjacent to one of the outside coaxial resonators 10a, 10c in order to provide the attenuation characteristic to the high frequency pass band. Through holes 11a, 11b, 11c, 111d are arranged along between the lateral surfaces in a series from one of side surfaces to the other one of the side surfaces.
A conductive material is coated onto the lower surface, the side walls, and an peripheral portion of the upper surface.
Respective coated upper areas 11-1, 11-2, 11-3, 11-4 as a loading capacitor pattern, coated with the conductive material are formed around the respective through holes 11a, 11b, 11c, 11d exposed on the upper surface of the dielectric block, and respective non-coated upper areas are formed between the coated peripheral portion of the upper surface and the coated upper areas of the resonators 10a, 10b, 10c and notch hole 10d.
An input pad 13a and an output pad 13b formed on one lateral side wall 22a are disposed adjacent to the upper surface to correspond to each of outside resonators. The input pad 13a and the output pad 13b are separated from the conductive material coated on the one lateral side wall 22a by non-coated lateral area 12a and non-coated lateral area 12b, respectively.
Non-coated lateral area 12a of lateral side wall 22a is coupled to non-coated upper area 14a of one of outside resonators 10a, 10c while the non-coated lateral area 12b of the one lateral side wall 22a is coupled to the non-coated upper area 14c of the other one of the outside resonators 10a, 10b, 10c, 10d.
An equivalent circuit of the dielectric filter of FIG 1A is shown in FIG 1B. In and out represent the input pad 13a and the output pad 13b. NR is defined by a diameter and a length of the notch hole. CN1 is defined by a distance between the loading capacitor pattern and the input and output pads while CN2 is defined by a distance between the loading capacitor pattern of the notch hole and the peripheral coated portion of the upper surface. FIG 1C shows reflection loss S11 and propagation characteristics or attenuation characteristics S21 of dielectric filter 10. An insertion loss P1 of the high frequency band is formed, and an attenuation pole AP of the high frequency band is established.
The conventional dielectric filters, however, are prevented from being reduced in size because the dielectric filters must be provided with additional volume for the notch hole. The conventional dielectric filters are relatively bulky in size compared to the minimized device which is installed with the dielectric filter 10. Therefore, I have found that the conventional dielectric filters are not reduced in size and that it is difficult and often inconvenient to install the conventional dielectric filter into the relatively small device in consideration of the recently developed minimized device.
It is an object to provide an improved dielectric filter having a plurality of resonators and input and output pads constructed according to the principles of the present invention.
It is another object to provide improved dielectric filter able to be reduced in size and to exhibit improved attenuation characteristics of a desired high frequency pass band.
It is yet another object to provide an improved dielectric filter able to remove a notch hole occupying an additional space in the dielectric filter.
It is still another object to provide an improved dielectric filter able to be mounted in a relatively small device which is installed with the dielectric filter.
It is a further object to provide an improved dielectric filter able to reduce a manufacturing cost.
It is also object to provide an improved dielectric filter able to shorten a manufacturing process.
These and other objects may be achieved by providing an improved dielectric filter having a notch pattern constructed according to the principles of the present invention. The dielectric filter includes a dielectric block, such as a dielectric ceramic block, defining an upper surface, a lower surface spaced apart from and being to the upper surface, two longitudinal lateral surfaces each spaced apart form each other and being parallel to both the upper surface and the lower surface, and two side surfaces disposed between the lateral surfaces and being perpendicular to the lateral surfaces. The lateral surfaces and the side surfaces form peripheral sides of the dielectric block between the upper surface and the lower surface. The lateral surfaces, the side surfaces, the lower surface, and a peripheral outside area of the upper surface are coated with a conductive material. Three resonators, such as first and second outside resonators and a middle resonator disposed between the outside resonators, includes through holes formed in the dielectric block, being parallel to each other, arranged between the lateral surfaces from one of the side surfaces toward the other one of the side surfaces in a series, and having respective openings exposed on the upper surface.
The conductive material is coated on each peripheral inner wall of the through holes. Also, the conductive material is coated around each opening of the through holes to form coated upper areas in order to provide a loading capacitance pattern to respective resonators. The coated upper areas are connected to the conductive material coated on the peripheral inner wall of the through holes. Non-coated upper area is formed around each of the coated upper areas of the resonators and between the peripheral outside area and each of the coated upper areas of the resonators.
Two non-coated lateral areas are formed on one of the lateral surfaces adjacent to the upper surface and coupled to the respective non-coated upper area of the upper surface. An input pad and an output pad are disposed within each of the non-coated lateral area of the lateral surface at a first position corresponding to the first resonator and a second position corresponding to the second resonator, respectively. The input pad and the output pad are spaced-apart from the conductive material coated on the lateral surface and to be electrically coupled to the first resonator and the second resonator, respectively, through the respective non-coated lateral areas of the lateral surface and the non-coated upper area of the upper surfaces.
One of the two non-coated lateral areas of lateral surface is continuously expanded along the lateral surface toward the other one of the two non-coated lateral areas, and one end of the expanded non-coated lateral area is disposed on a third position of the lateral surface corresponding to the middle resonators in order to provide a capacitance between the input pad and the middle resonator. The extended non-coated lateral area is separated from the non-coated upper area of the middle resonator by a conductive material covering between the extended non-coated lateral area of the lateral surface and the non-coated upper area of the upper surface.
The notch pattern made of the conductive material and disposed within both the non-coated lateral area and the extended non-coated lateral area of the one of the lateral surfaces is continuously extended from the input pad toward a middle position corresponding to the middle through hole of the middle resonator along the lateral surface in a direction parallel to the upper surface in order to provide a capacity coupling between the middle resonator and the input pad.
With the existence of the notch pattern, an electric field is formed between the middle resonator and the input or output pad. Since the attenuation pole is formed on a desired high frequency band in accordance with the capacitance equivalent to the electric field formed between the input pad and the middle resonator, the attenuation characteristic is established in the desired high frequency band.